Electroluminescent lamp



Oct. 27, 1964 E. c. PAYNE 3,154,712

ELECTROLUMINESCENT LAMP Filed May 19, 1952 United States Patent Oarsenic ELESTRGLUNT nSCENT LABT? Elmer C. Payne, Binghamton, NX.,assigner to Sylvania Electric iroduets inc., Salem, Mass., a corporationof Massachusetts Filed May 19, i952, No. 233,62@ 7 Claims. (Qi. 3io-NS)This invention relates to lamps in which light is produced by theapplication of an electric iield to a region including a phosphor. Theinvention relates also to phosphors particularly suited for such use,and to methods of preparing them.

Electric lamps utilizing phosphors have been previously known. ln onecommercial type, the electric iield is applied to a gas and theresultant ultraviolet radiation used to excite the phosphor. ln another,the iield is applied to an evacuated space in which electrons areaccelerated and the resultant cathode ray beam used to excite thephosphor. But in neither of these devices is the light produced by thedirect application of a suiliciently intense field to the immediateregion of the phosphor.

Moreover, both of these devices, like the well-known incandescent lamp,require an enclosing bulb or tube hermetically sealed, with consequentdiiliculties in manufacture and limitations in geometry. The devices ofmy invention are far less limited in such respects, requiring no sealedbulb or tube for their operation, and can be made in practically anyconvenient size and shape, thus making possible for the first time suchnew lighting applications as the illumination of a room by a completeceiling of electrically-luminous plaques.

As in certain specic embodiments of the invention described below, suchplaoues `advantageously have a plate of conductive glass in contact withone side of a thin layer of phosphor embedded in a light-transmittingdielectric material, and a conducting layer in Contact with Ithe otherside of said phosphor layer.

The device has a positive volt-ampere characteristic and hence requiresno ballast. lt can be operated directly from the usual 110 volt,60-cycle per second power line, although in some cases a transformer maybe desirable for higher voltage operation.

The device is in effect a luminous capacitor, and the light producedappears to be due to the action of the electric field on the phosphor,or on the phosphor and the embedding material.

The appearance of a faint glow upon the application of an electric heldto a phosphor film appears to have been observed heretofore, but only asan obscure scientiiic phenomenon. Attempts by earlier scientists toproduce light of practical illuminating intensities by such means appearto have been unsuccessful, as pointed out by Leverenz in hisrecently-published book, An Introduction to the Luminescence of Solids(McMillan, New York, 1959, page 290). Leverenz found that excitation ofallegedly held-excited phosphor lrns produced only a feeble blue glow,barely perceptible to a partially darkadapted eye, and he attributed theglow to excitation of aerospheres of atmospheric nitrogen trapped in theembedding medium, rather than to excitation of the phosphor itself bythe field.

In contrast to this, the light produced by the present invention is notat all the blue glow of atmospheric nitrogen, and can be made blue,green, yellow, red or other color depending on the choice of phosphor.And instead of the faintly perceptible glow of the films examined byLeverenz, my lamps can have brightness as high as foot-lamberts or more.This brightness can be maintained over large areas to yield a very largetotal light output.

rThese unexpected results appear to be due to direct excitation of thephosphor by the field. I have discovered that useful electroluminescentphosphors generally comprise a host crystal containing not only anactivator impurity, but also a donator impurity, the latter providingelectrons wmch can be excited into the conduction band of the crystal,with the expenditure of a small energy; and from there can beaccelerated by the iield until they have enough energy to excite theactivator atoms. For example, one very effective electroluminescentphosphor comprises zinc sulfide with a copper activator impurity and alead-donator impurity.

Copper-activated Zinc sulde containing lead has been known as aninfra-red responsive phosphor, but the amount of lead used for suchinfra-red phosphors is considerably more than can be used in an eiicientelectroluminescent phosphor. The amount of retained lead in infra-redphosphors is greater than the highest amount which gives appreciableelectroluminescence. An infrared phosphor does not ordinarilyelectroluminesce, and an electroluminescent phosphor does not ordinarilygive infra-red response.

Once an electron is freed from a donator atom in the crystal, it must beaccelerated through a considerable number of atom spacings by the iield.For example, with 600 volts across three-thousandths of an inch layer ofphosphor, the two-volt drop necessary for visible emission may require adistance of about 2.5 X10-5 centimeters, or about 2000 atom-spacings.The mean free path in looselypacked crystals such as zinc sullide, underthe influence of a iield, may be itself as great as 10-5 or more, sothat the mean free path and the required distance of travel may not begreatly different. The electron can travel for a considerable number ofmean free paths, if the energy losses at the end of each path are small.Loosely packed crystals such as the sultides, selenides, and silicatesof cadmium, zinc, and the like, have large mean free paths and are thussuitable for electroluminescence, when -a suitable activator and donatorare present.

When the iield is suciently high, it may produce free electrons fromatoms, by shifting electrons directly from a lower band to theconductive band, and the presence of some famt electroluminescence maybe obtained under such circumstances without a donator impurity, but theaddition of a donator impurity will greatly increase the light emissioneven in such cases. For example, in one copper-activated, lead donator,zinc sulfide phosphor, the light emission was increased from a barelyvisible illumination of 3 units to a bright illumination of 460 units bythe addition of only 0.001% lead by Weight.

ln the speciiic embodiments of the invention described below, thephosphor is placed between two conductors across which a voltage can beapplied. In a preferred embodiment, a thin layer of a dielectricmaterial having me crystalline phosphor particles embedded therein iillsthe space between two electrically conducting layers, at least one ofwhich is light-transmitting such as a conducting glass or plastic. lnthe absence of the embedding materi l, the crystals appear to glow onlyat their points of contact with a conductor. The presence of theembedding medium greatly enhances the glow, which can be seen under themicroscope to spread out over the entire crystal. The phosphor in itsembedding dielectric appears to be excited directly by the field.

The response of a particular phosphor to this method of excitationcannot be -redicted from its response to other forms of excitation. Someof the best iluorescent lamp phosphors do not resnond to the electricfield, but the zinc sulphide type phosphor can be made very eiective forsuch purposes. The crystals of one effective zinc sulphide type phosphorappear to have small bumps on at surfaces, and those bumps may besmaller crystals, of zinc oxide aihxed to the main zinc sulphidecrystals, or they may be due to some other distor'tl'ng inliuence. Ifthe bumps or projections are zinc oxide crystals, then there may be avery high iield at the interface between these and the main zincsulphide crystal. And if they are not Zinc oxide, the bumps may stillincrease the iield because of their distortion of the crystal shape. Inany event, the oxide, the activators and other impurities added to thecrystal would seem to insure suliicient irregularity inside the crystalto alford the possibility of localized build-up of the iield at somepoint in the crystal.

' A certain amount of zinc oxide is helpful in the zinc sulphide mixturewhile the latter is i'lred, presumably because it aids in formation ofthe proper type and consti-tution of crystals. Nonetheless washing thephosphor with a suitable solvent for zinc oxide, such as a solutionofacetic acid or `of ammonium acetate, improves and multiplies thebrightness of such phosphors. Such solvents may also have the eifect ofremoving surface impurities from the surfaces oi the active crystals,and the latter may actually be the determining factor, but in any casethe solvents do remove the excess zinc oxide which is of lowresistivity. This has the efiect of increasing the resistivity of thephosphor about 100 times in some cases and the resultant highresistivity improves the phosphor.

The emission from the sulphide phosphors may be a surfaceV eifect, ordue to a surface layer between the dielectric medium and the mainportion of the crystal, but the great brightness of these highresistivity phosphors may indicate that the iield penetrates well intothem and excites a larger portion of the crystal material than would be-the case with phosphors of lower resistivity.

Other features and advantages of the invention will be apparent Vfromthe following detailed description of specific embodiments thereof.

In the accompanying drawings, three devices embodying aspects of theinvention areshown, FIG. 1 being a perspective -view partly in sectionof one such device; FG. 2 being a perspective view of a second device;FIG. 3 being 2in-enlarged cross-sectional View of the device in FIG. 2;FIG. 4 being a plan cross-sectional view of a third device along theline 4 4 in FlG. 5; FIG. 5 being an enlarged cross-sectional view of thedevice in FIG. 4, along the line 5 5, and FIG. 6 being an energy diagramof a phosphor according to the invention.

VThe device shown in FIG. 1 has a glass plate 1,

having -a transparent conductive surface 2, over which is a thin layer 3of phosphor-impregnated dielectric material, with a metal backing layer4 over that and in intimate contact therewith. This completes anillumination source, suitable for use as a luminous plaque for walls andceilings, for example. One terminal of a prope source of varying oralternating voltage can be connected to the metal backing layer d, theother to a metal tab 5 which is connected to the conducting surface 2.

In a modification of this device the layer 4 can also be of conductiveglass, instead of being of metal, thus providing a plaoue which emitslight from both sides, and when not energized is translucent. Such adevice can be used in various ways, for example in table lamps andotherlighting iixtures or even as a window pane which transmits sunlightby day and emits its own light at night.

A conducting surface 2 of good transparency or translucency is difficultto obtain, because good electric conductors are generally goodreflectors of light, rather than Vso transmitters of it. However,although other coatings may be used, I iind that a particularlyeffective conductive surface lmay be provided by heating the glass andexposing it while hot to vapors of the chlorides of silicon, tin, ortitanium, and afterward placing the treated glass in a slightly reducingatmosphere. Where the application in the vapor state is not convenient,good results i may be obtained by mixing stannie chloride with absolutealcohol and glacial acetic acid and dipping the glass plate into themixture.

However applied, the resultant conductive surface appears to containstannic (or silicic or titanic) oxide, probably to some extent at leastreduced to a form lower than the dioxide, although the exact compositionis diliicult to determine.

The conductive surface 2 so applied will have a resistance of about ohmsper square, that is a resistance of 100 ohms taken between the entireopposite sides of any square on the surface 2.

TheV phosphor-impregnated layer 3 placed over the transparent conductivelayer 2 is a phosphor of copperactivated zinc sulphide as describedbelow, in the form of line particles embedded in plasticizednitro-cellulose.

For example, about 20 grams of nitrocellulose with a suitableplasticizer such as chlorinated diphenyl can be dissolved in about 8Occ. of a suitable solvent such as butyl acetate, and about l0 grams offinely-divided phosphor suspended therein. The plasticizer in the aboveexample is included in the weight of the nitrocellulose, which can be ofquarter-second viscosity, although any convenient viscosity can be used,the proportions and constituents of the above mixture being capable ofconsiderable variation. A large number of plasticizers are well known,but for best results those with high resistivity and high dielectricconstant should be chosen. The plastieizer may be used in considerablequantity, if desired, and may even comprise the major portion of thecombined weight of nitrocellulose and plasticizer, as shown in theapplication of Eric L. Mager tiled concurrently herewith, and a non-aciddielectric medium, or one of 10W acidity or acid number may be used asshown in that application.

The backing layer 4 is of metal, preferably a good retiecting metal suchas aluminum or chromium, which will not react appreciably with thephosphor or embedding material used. The metal layer or conductivesurface d is preferably of low resistance and can be applied in anyconvenient manner, taking care not to damage the cellulose-phosphorlayer. However, best results have been obtained by vacuum-deposition ofthe metal. The glass plate'll, with its conductive surface 2, is coatedwith the embedded phosphor layer 3, placed in a bell jar and the latterevacuated. The coating 3 is then heated for a moment, for example bypassing a current through the conductive surface 2. The heating ispreferably ofrthe order of that used for drying, and should not, ofcourse, be sutiicient to char the embedding material 'inphosphor layer3. The heating is not essential to producing a plaque of good initialbrightness, but aids in maintaining the brightness throughout the lifeof the lamp.V

The aluminum or other metal'is then deposited on the phosphor layer in avacuum, for example by being placed on a tungsten filament and thelatter heated by the passage of an electric current therethrough, asshown for example in U.S. Patent 2,123,706, issued July 12,

1938, to O. H. Biggs.

Various other plastics can be used instead of nitrocellulose. Glass andvarious enamels may be used, particularlyV glass of low enough meltingpoint to insure that the phosphor crystals remain unmelted.

The thickness of the various layers can be altered to suit` variousvoltage conditions and the like. The voltage necessarily will depend onthe phosphor used, the thickness of the phosphor layer 3, and thebrightness desired, but voltagesbetween'ZS'volts and 2500 volts and evenhigher have been used. A lamp operable from a 11G-volt alternatingcurrent power line can be made with the conducting surface 2 of athickness of about a wavelength of light, producing an iridescent effectwhen viewed at an angle, the phosphor layer 3 of about 2 onethousandthsYof an` inch, and the metal layer 4 of a fraction of a thousandth of aninch. The plate 1 can have any convenient thickness and should betransparent or translucent.

A highly eective phosphor can be prepared by intimately mixing as tinepowders about 75 parts by weight of zinc sulphide and parts zinc oxide,with about 1.0 part zinc chloride, about .075 part copper added ascopper sulphate, and about 1 part lead sulphate.

These are the preferred values for best results, but the copper,calculated as metallic copper, can be varied over a range of about 0.03%to 0.3%, and the amount of chloride, calculated as zinc chloride, shouldbe between 0.4% to 2.0%, although if a fluoride is used the amount addedshould not ordinarily be greater than about 0.1%. The amount of lead,calculated as the sulphate, should be between and 5%, the higher amountsbeing used only when there is a considerable flow of itrogen or otherinert gas during the firing, to carry away the excess lead. The amountof lead retained in the nal phosphor should be only between about 0.01%and 0.000l% by weight for best brightness.

The components should be thoroughly mixed in the form of line powders,and heated to between 900 C. to 1250 C. in an inert atmosphere, forexample in a gastight electric furnace lled with nitrogen, and having achamber of nitrogen connected thereto to pass a sloW stream of the gastherethrough. A batch of 200 grams has been tired in an electric furnaceat about 1000" C. in a small silica boat in a silica tube 3 inches indiameter and inches long sealed at both ends, with a quarter inch silicatube feeding nitrogen into one end of the tube and a quarter inch tubefor exhausting the nitrogen at the other end. The rate of flow or"nitrogen was about 0.1 liter per minute.

In tiring the phosphor, there are three distinct stages. In the lirststage, the mixture emits fumes of the halide and the lead compound used,and turns a yellow color, which deepens with heating. lf removed fromthe furnace at this stage, the material will not have any appreciableelectroluminescence. In the second stage, the evolution of fumesdecreases greatly and the color of the phosphor darkens somewhat to agreenish-gray. If removed from the furnace during this stage thephosphor will luminesce. Finally there is a third stage, where thephosphor darkens further and becomes gritty, gradually losing itselectroluminescent capability. The phosphor should be removed from thetiring furnace toward the end of the second stage or the beginning ofthe third. The soft, tlutly mass can then be crumbled or shaken toseparate the powder particles.

After tiring of the phosphor, a treatment with acetic acid or ammoniumacetate improves the luminescence of the phosphor. ri`he brightness isusually increased several thnes, and in many cases the treatment makesthe difence between a good brightness and no brightness at in treatingthe tired phosphor powder with acetic acid, a solution of about 5% ofthe acid in Water is heated to between 60 C. and 100 C., preferablynearer to 60 C., and poured over the phosphor while the latter issubjected to a gentle grinding action until thoroughly treated, forexample, about 2 minutes, and the suspension is then filtered, washedwith Water, and dried. The temperature of the solution is kept at about60 C. to 100 C. during the entire treatment, even during the iiltering.

lWhile the foregoing treatment improves the phosphor, an ammoniumacetate treatment may be preferred because it is less critical in useand is more effective in increasing the brightness. In some cases, theacetate has given a 50% brighter phosphor than the acetic acid. ln usingthis treatment, enough of a saturated solution of ammonium acetate inwater is added to the phosphor to rgive a slurry, which is stirred in amotar and thoroughly ground until all the large aggregates are broken upinto their component particles. Then a quantity of half-saturatedacetate solution is added, in proportions of say 200 cc. for every gramsof phosphor, enough to give a thinner slurry and the supernatantsuspension poured oir". A similar amount of half-saturated acetatesolution is then added to the phosphor arid poured off or iiltered oli.The process is repeated with successive dilutions, two ltreatments withone-eihth saturated solution, then two with one-twelfth saturation, twowith onesixteenth and several with pure water. The treatment could, ifdesired, be continuous with two streams of liquid, one being of waterand one orF acetate solution, pouring onto :the phosphor, the stream ofacetate solution being gradually reduced in ilow. if the acetateconentration on the phosphor is not diluted gradually, the dissolve zincoxide will precipitate out again over the phosphor.

The eiectiveness of the treatment is apparently in removing the excesszinc oxide, leaving the sulphide and presumably leaving also any smallparticles of zinc oxide Whh may have attached themselves to thesulphide, or any zinc oxide distributed throughout the sulphidecrystals.

The treatment is also found to remove a considerable fraction of thelead.

if the acetic acid solution used, it should be Weak enough to remove theoxide without also removing the sulphide. With the :acetate solution,the sulphide is unaffected, and the pl-i of the solution may be variedfrom 4 to 9, by varying the proportions o the ammonium acetate radicals,and still be edective. This is a helpful circumstance, for commercialammonium acetates generally vary in composition, diering considerablytrom stoichiometric. The acetate solution may possibly remove also anysurface lm on the sulphide crystals and perhaps some of the copper,although the latter seems less likely.

Other ammonium sts, such as the chloride, are also effective, butammonia itseli is not satisfactory. The reason appears to be that in areaction between zinc oxide and ammonium acetate a complex zinc diammineacetate is formed, plus water. Ammonia itself lacks a negative radicalto form such a complex for removing both the oxide and zinc portion 'ofthe Zinc oxide. But ammomurn chloride and many other ammonium saltswould worl: like the acetate.

The effects of the 4treatments described on a ZnS-ZnO phosphor, suitablyactivated, are indicated in the following table:

Relative Brightness Percent ZnS in Starting Mixture Untreetod TreatedThe treatment in the above tests was with acetic acid. the ammoniumacetate treatment generally gives about 50% more brightness near themaximum point.

it is seen from the table that the treatment had no effect on the 100%zinc sulphide sample, presumably because there was in that case no Zincoxide to be removed, but in the other cases the treatment had aremarkable effect in increasing the brightness. The untreated samples-appeared to have no appreciable luminosity before treatment and thismay have been due to the screening effect of the zinc oxide, which hasan electrical conductivity high with respect to that of the sulphide.

The test device actually passed 100 times as much current with untreatedphosphors as with treated ones. With a sinusoidal 100 volts at 60 cyclesper second on a test device using `a cell 0.01 inch thick and 5 sq. in.in area, between a metal plate at the bottom and a piece of conductiveglass at its top, using 1.5 grams of phosphor in 1.2 cc. of castor oil,the current passed was milliamperes before treatment and 0.05milliampere after treatment. Thus a high-resistivity phosphor is demedfor the purposes of this specification as one which passes of the orderof 0.05 milliampere when tested in the above apparatus under theconditions specified, that is one which does not pass more than 0.5milliampere.

The resistivity of the treated phosphor is even greater than the currentpassed throughtne cell might at rst seem :to indicate, because a largepart of the current through the cell is due to capacitance. The currentof 0.05 milliampere when the cell is iilled with the castor oil andtreated phosphor in the above example is about double that passe whenthe cell is lilled with castor oil alone. Since the phase angle betweenvoltage and current is only about tive or ten degrees, only a small partof the increase in current is due to the conductivity of the treatedphosphor. The greater increase appears to be due to the increaseddielectric constant of the mixture with the treated phosphor added.Since the dielectric constant of the phosphor-oil mixture appears to bedouble that of the oil alone, the constant of the treated phosphorappears to be quite high, say greater than ten, because the phosphormakes up only about a third of the volume of the mixture in the cell.

But when the phosphor is untreated, there is a considerableV amount ofzinc oxide present, and since the dielectric constant for the oxideappears to be only about 2.5, the great increase in current with theuntreated phosphor is due to its high conductivity.

A further example of a phosphor useful in electrolurninescent deviceshas been prepared by intimately mixing the following ingredients asiinely-divided powders,

in the proportions indicated:

Grams Zinc sulphide (ZnS) 75.60 Zinc oxide (ZnO) 24.42 Lead carbonate(PbCO3) 1.37

Cuprous oxide (CuO) 0.0637

The sulphide used contained 5 grams of water and 1% zinc chloride. rl`hewater was removed by drying the mixture at'160 C. The batch was thenplaced in a quart Vmill and milled with acetone for half an hour, afterwhich it was again dried and then tired at 1000 C. in an open silica trafor half an` hour in a furnace. Pre-puried nitrogen was iiowed over themixture in the furnace at a rate of 0.06 liter per minute, during thetiring. After this heating, the phosphor was ground lightly in a mortarto break up the resultant fluly cake into its component particles orinto its smaller aggregates of particles.

The powder was then treated with a boiling solution of 5% acetic acid inwater, Ithen with a 1/2 solution of the 'same and then washed withwater. In each case the phosphor Was placed in the acetic acid solutionand the whole raised to boiling temperature in about ten minutes,continuing the boiling for five minutes.

In phosphors using about oxide in the starting mixture, the percentagehas been found to be still substantially 25% after iiring. But aftertreatment with the solutions as above described, the Aamount of oxidepresent has been reduced to 5% or less, so that the iinal phosphor isabout 95% sulphide or more.

Phosphors made Without any oxide in the starting materials haveluminesced but were only about 20% as bright las those made with oxide.The copper content for even the 20% brightness should be much greaterthan the optimum values for the phosphors using oxides. A small amountof oxide may possibly be formed in this phosphor during firing, since itis diicult to insure that every trace of 'oxygen is absent from thenitrogen `atmosphere used during tiring.

The sulphide phosphor prepared as in the preceding examples iluoresces agreenish-yellow under excitations b at 60 cycles per second andfluoresces blue under excitation of about 2000 cycles.

A phosphor uorescing orange-yellow may be prepared by using manganese inactivating amounts with the 'sulphide. In an example, such a phosphor isproduced by mixing 87 parts by weight of zinc sulphide and 13 parts zincoxide as finely divided powders, together with about 2.1% manganoussulfate and 0.8% zinc chloride and 1% lead sulfate. The constituents areblended as dry powders, preferably finely-divided, or they may beblended wet, for example, in `a water slurry and dried. Some of thecompounds will remain yas powders and some will dissolve in the Water.

In any case, after the components 4are thoroughly mixed together, theresultant mixture should be fired, preferably in an inert atmosphere, aswith the previously-described phosphor, .at a temperature of about 900C. to 1200 C., preferably about 1000 C. After firing, the resultant massis crumbled or milled to a desired particle size, although tne less themilling, the better will be the phosphor.

The phosphor should then be given a 'treatment such as the acetic acidor ammonium acetate treatment previously described, to improve itsbrightness. Such a treatment appears to be less marked in its effect onthe manganese phosphor, probably on account of the smaller quantity'ofoxide in the starting mixture.

Among the other manganese-activated phosphors which exhibitelectroluminescence are the cadmium silicate and zinc fluoridephosphors. Electroluminescent sulphide phosphors can also be made inwhich the zinc is replaced partly or entirely by calcium or strontium.

FIGS. 2 to 5 show forms of the invention in which paired long spacednarrow conductors 6 and 7, 8 and 9, are placed side by side, theconductors and the space between them being occupied by -a coating orlayer 10, 11 consisting of an electroluminescent phosphor embedded in adielectric material. The conductors and the layer `are carried byinsulating supports 12 and 13. In FIG. 2 the conductors 6, 7 are wires,having an enamel insulating layer 14, Wound side by side `and closetogether but spaced apart a distance of a few thousandths of au inch orless.

A lamp is defined for the purposes of this specification as a devicewhich produces light of practical illuminating intensities. YIntensitiesbelow =a foot-lambert are practical for some application, although thelamps herein described have given several foot-lamberts on 60 cycles persecond alternating voltage supply, and 15 to 20 foot-lamberts on asupply of several thousand cycles per second.

Such lamps are therefore useful for general illumination purposesincluding use as luminous panels for ceilings, as lighting sources fortable lamps, as luminous signs and clock faces, as luminous face platesfor household electrical switches, for street lighting `and for manyother applications.

A typical energy level diagram for an electroluminescent phosphor isshown in FIG. 6. Between the highest lilled band 15 of the host crystaland the lowest unlled band 16, there is an energy gap 17. It thephosphor is to emit visible light, the width of 'this gap must begreater than 1.5 electron volts, which corresponds to the energyequivalent to the extreme red end of the Visible spectrum, that is tothe lowest energy which can produce visible light. In zinc sullide thegap is about 3.5 volts.

The highest occupied band 18 of the activator material will be spacedfrom the lowest unfilled band 16 by an yamount less than for the gap 17,Iand in the case of copper in zinc suliide will be spaced about 2.7volts. The donator impurity will have its highest occupied level 19 justbelow the bottom of the lowest unfilled band 16, .and the gap betweenthe two Will generally be only a few tenths of a volt, for examplebetween -about 0.1 volt and 0.4 volt for a lead impurity incopper-activated zinc sulfide. Other activating materials can be used,for example silver atea-,712

or manganese, and the most suitable material will be different fordilerent host crystals. A host material ot fairly open crystal structureand consequent long mean free path facilitates the acceleration of theelectrons to a value suitable for exciting the activator material.Metailic sulphides, selenides fand silicates are among the comA poundswhich can be used as host crystals. Zinc, cadmium, and calcium are amongthe substances which can be used as the metallic component or thecompounds.

A donator impurity is one from which electrons can be treed by thermalagitation or by the field at relatively low energy values, generally notover a few-tenths of a volt. A donator impurity is a substance having ahigher valence than one of the atoms or ions of the host crystal so thatit can substitute for such an atom or ion, and lose an electron in theprocess. The lost electron will take up a low energy orbit of largeradius around the atom or ion from which it has thus been partly freed,and may then be more nearly completely freed by a small additionalenergy, the amount supplied by thermal agitation often being suicient.For example, lead, thallium, indium, and tin are among the metals whichcan replace par-t of the zinc in zinc sulphide and act as donatorimpurities.

For best brightness, the amount of donator material present should beless than about 0.00005 gram-atom per mole of host crystal material,although with some phosphors electroluminescence can be obtained withtwo to three times that proportion of donator material. The amount ofdonator material should, of course, be greater than zero and for bestbrightness should generally be above 0.0000005 gram-atom per mole ofhost crystal material.

When chlorine is used in the starting mixture from which the phosphor ismade, the amount retained in the tinal Washed phosphor will notordinarily be greater than 0.1% by weight of the final washed phosphor.

Activators or the types held in part at least interstitially in thephosphor are particularly eiective, and the amount of such activatorsused is much greater than the amount ordinarily used in cathode ray orultaviolet excited phosphors, or in infra red-responsive phosphors, asshown by the comparatively high copper content of the electroluminescentzinc sulphide phosphor described above.

The highest occupied band 19 is generally known as the Valence band andthe lowest uniilled band 16 as the conduction band.

This application is in part a continuation of my abandoned applicationsSerial Nos. 105,803, 119,021, and 119,022 and Patent 2,838,715, iiledrespectively July 20, 1949, September 30, 1949, September 30, 1949, andAugust 22, 1950.

What 1 claim is:

1. A high resistivity electroluminescent phosphor consitsing essentiallyof a host crystal of Zinc suliide, an activating copper impuritytherein, said copper being present in an amount between 0.03% and 0.3%of the weight of zinc sulfide, and a donator impurity of lead therein tofurnish electrons for excitation of the activating impurity, said leadbeing present in an amount between about 0.0001% and 0.01% by weight ofthe zinc suliide.

2. An electroluminescent phosphor consisting essentially of a hostcrystal of zinc sulfide, an activating impurity therein, and a donatorimpurity therein to furnish electrons for excitation of the activatingimpurity, said donator being present in an amount between 0.0000005 and0.00005 gram atoms per mol of zinc sulfide, and being selected from .thegroup consisting of lead, thallium, indium and tin.

3. An electroluminescent lamp comprising a first electrode, a secondelectrode in proximity thereto, and a solid layer therebetween includingan electroluminescent phosphor consisting essentially of activatedcrystals of combined zinc oxide and sulphide and substantially free fromany uncombined zinc oxide, and in which the phosphor contains a smallamount of lead.

4. The lamp of claim 3, in which the phosphor comprises a fired mixtureof zinc sulphide, zinc oxide, a copper compound and a lead compound.

5. The lamp of claim 4 in which a halide is one of the ingredients ofthe mixture tired.

6. Electroluminescent capacitor phosphor crystals, each said phosphorcrystal having a plurality of separated conductive coats each in aseparate part thereof and arranged in capacitor structure.

7. The capacitor-phosphor crystal of claim 6 and a dielectric coveringoverall.

References Cited in the tile of this patent UNITED STATES PATENTS2,136,871 Wakenhut Nov. 15, 1938 2,447,322 Fonda Aug. 17, 1948 2,474,506V/Ood .lune 28, 1949 2,559,279 Charles July 3, 1951 2,566,349 MagerSept. 4, 1951 2,624,857 Mager June 6, 1953 2,745,811 Butler et al May15, 1956 FOREIGN PATENTS 873,860 France July 22, 1942 OTHER REFERENCESArticle by Destriau in the Philosophical Magazine, vol. 38, October1947, pages 700 to 723.

3. AN ELECTROLUMINESCENT LAMP COMPRISING A FIRST ELECTRODE, A SECONDELECTRODE IN PROXIMITY THERETO, AND A SOLID LAYER THEREBETWEEN INCLUDINGAN ELECTROLUMINESCENT PHOSPHOR CONSISTING ESSENTIALLY OF ACTIVATEDCRYSTALS OF COMBINED ZINC OXIDE AND SULPHIDE AND SUBSTANTIALLY FREE FROMANY UNCOMBINED ZINC OXIDE, AND IN WHICH THE PHOSPHOR CONTAINS A SMALLAMOUNT OF LEAD.